Reference-less clock circuit

ABSTRACT

A programmable reference-less oscillator provides a wide range of programmable output frequencies. The programmable reference-less oscillator is implemented on an integrated circuit that includes a free running controllable oscillator circuit such as a voltage controlled oscillator (VCO), a programmable divider circuit coupled to divide an output of the controllable oscillator circuit according to a programmable divide value. A non-volatile storage stores the programmed divide value and a control word that controls the output of the controllable oscillator circuit. The control word provides a calibration capability to achieve a desired output frequency in conjunction with the programmable divider circuit. Open loop temperature compensation is achieved by adjusting the control word according to a temperature detected by a temperature sensor on the integrated circuit. Additional clock accuracy may be achieved by adjusting the control word for process as well as temperature.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.11/277,600, filed Mar. 27, 2006, entitled “Reference-Less ClockCircuit,” naming Augusto Marques as inventor, which application claimsthe benefit of provisional application 60/777,146, filed Feb. 27, 2006,naming Augusto Marques as inventor and entitled “Crystal-less ClockCircuit,” which applications are incorporated herein by reference intheir entirety.

BACKGROUND

1. Field of the Invention

The invention relates to oscillator circuits and more particularly tooscillator circuits operating without a reference source such as acrystal.

2. Description of the Related Art

A significant percentage of the clock chips offered in the market todayare crystal oscillator based. These solutions can normally achieve verygood frequency stability, e.g., on the order of ±10 parts per million(ppm). However, such clock chips demand the use of a crystal and needhermetic packaging, which creates additional expense. Furthermore, notall applications require that type of frequency accuracy.

SUMMARY

Accordingly, the invention provides in one aspect a programmablereference-less oscillator circuit that provides a wide range ofprogrammable output frequencies. The programmable reference-lessoscillator may be implemented on an integrated circuit that includes acontrollable oscillator such as a voltage controlled oscillator (VCO)arranged in an open loop configuration. A programmable divider circuitis coupled to divide an output of the controllable oscillator circuitaccording to a programmable divide value. A non-volatile storage storesthe programmed divide value and a control word that controls the outputof the controllable oscillator circuit. The control word provides acalibration capability to achieve a desired output frequency inconjunction with the programmable divider circuit. Open loop temperaturecompensation is achieved by adjusting the control word according to atemperature detected by a temperature sensor on the integrated circuit.

In an embodiment, a method is provided for operating an integratedcircuit. The method includes determining a control signal for acontrollable oscillator, at least in part, according to a stored valuein a non-volatile memory, supplying the control signal to thecontrollable oscillator, thereby determining an output of thecontrollable oscillator, and dividing the output of the controllableoscillator in a divider circuit. A divide value for the divider circuitmay also be stored in the non-volatile memory. Open loop temperaturecompensation is provided by adjusting the control signal according to atemperature sensed in a temperature sensing circuit. The method mayfurther include adjusting the control signal according to both thedetected temperature and one or more process parameters associated withthe integrated circuit.

In another embodiment a method is provided for making a reference-lessoscillator that provides a selected output frequency. The methodincludes programming a divide value for a divider circuit that dividesan output of a controllable oscillator circuit, the output signal of thedivider circuit having a frequency that differs from the selected outputfrequency. A control value that controls the output of the controllableoscillator circuit is varied until the output from the divider circuitis the selected output frequency. That control value is stored innon-volatile memory. The divide value is also stored in the non-volatilememory. The method may further include storing information in thenon-volatile memory relating to adjusting the control value according toa detected temperature. The method may further include storing atemperature detected by a temperature sensor of the reference-lessoscillator at the time the reference-less oscillator is supplying theselected output frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 illustrates a block architecture diagram of a reference-lessoscillator according to an embodiment of the invention.

FIG. 2 illustrates how all frequencies within a range can be achieved.

FIG. 3 shows a graph of frequency versus temperature versus processillustrating various aspects of calibration and temperature compensationas well as process variations associated with embodiments of theinvention.

FIG. 4 illustrates a floor plan of an exemplary oscillator circuitaccording to an embodiment of the invention.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

There are a large number of applications utilizing clock chips for whichan accuracy of ±100 ppm is sufficient. Described herein is anarchitecture to build a clock chip, without using a reference sourcesuch as a crystal or surface acoustic wave (SAW) oscillator, that canachieve suitable accuracy on a fully integrated silicon solution,without the requirement for hermetic packaging and that can bemanufactured at a very low cost. In one preferred embodiment, the diesize is small (e.g., <0.5 mm²). A preferred embodiment may also includelow pin counts and be compatible with existing crystal oscillatorpackages and pin-outs. For example, in an embodiment the integratedcircuit may have VDD, GND, XOUT and ENABLE pins, where VDD and GND arepower and ground, respectively, XOUT is the clock out signal, and ENABLEenables the clock out signal. Note that term “pin” is used togenerically refer to any sort of connection provided between theintegrated circuit and an external signal. Other embodiments may haveadditional or few input and output pins.

Some embodiments may utilize one of those pins, e.g., the ENABLE pin,for serial communication with the crystal-less oscillator circuit forprogramming/calibration purposes. Other embodiments may provide aseparate serial communication port. For example, an embodiment mayprovide a separate serial communication including SCLK, SDI and SEN,where SCLK, SDI, and SEN are serial port clock, data, and enablesignals, respectively. Any other suitable one-pin or multi-pin serialcommunication port may be utilized.

In addition, it is possible utilizing the teachings herein to provideone die capable of generating all frequencies within a particular range,e.g., up to a few hundred MHz. Furthermore, the architecture presentedherein allows the output frequency to be digitally defined or controlledinstead of requiring a different crystal for different frequencies. Theclock frequencies may be determined at the factory and stored innon-volatile memory.

FIG. 1 illustrates a block diagram of a reference-less clock chip 100according to the embodiment of the invention. At the core of the systemis a free running voltage controlled oscillator (VCO) 101 temperaturecompensated in an open loop configuration. The VCO 101 supplies itsoutput to a programmable divider circuit 103. The illustrated embodimentincludes a voltage regulator circuit 115 providing a stable power supplyto the VCO 101. A digital to analog converter (DAC) 105 receives acontrol word from control logic 109 to fine tune the output of the freerunning VCO 101. The value of that control word is determined duringcalibration and stored in non-volatile memory 111. The value of thatcontrol word may be modified based on temperature during operation.Temperature sensor 113 supplies a sensed temperature value to analog todigital converter 107, which converts the sensed temperature to adigital value. The digital value representing the sensed temperature isused to adjust the control word supplied to DAC 105. Various types ofoscillators may be used as the free running oscillator 101. Oneembodiment utilizes a bond wire LC based oscillator circuit. Anotherembodiment may utilize an integrated inductor LC based VCO. An RC basedVCO architecture or a ring oscillator based architecture may also beused. Thus, the free running oscillator 101 is a controllableoscillator, typically a VCO, but other suitable oscillators may beutilized. The controllable oscillator may be incorporated directly intothe die or a portion of the oscillator (e.g., the inductor or a portionthereof) may be formed external to the die and included in the packagedintegrated circuit.

The VCO 101 can operate from a frequency (fo) to (fo+Δf), where Δfrepresents the tuning range. By dividing down, one can generatefrequencies (fo/N) to (fo+Δf)/N. If N is made programmable, then one canpotentially generate all possible frequencies within a particular range.With reference to FIG. 2, in order to generate all possible frequencies,the tuning ranges need to overlap: fo/(K−1)≦(fo+Δf)/K. That isΔf/fo≧1/(K−1). So in order to have a small VCO tuning range, whichsimplifies the VCO, a large K, i.e., a large divider value N, isdesirable, and therefore a high VCO frequency. For example, in anexemplary embodiment, the voltage controlled oscillator (VCO) is run at3 GHz and it is desired to generate up to 100 MHz output clocks fromoscillator circuit 100. The minimum divide value N is therefore 3000(MHz)/100 MHz=30. That yields a tuning range Δf/f≧ 1/30 or 3.33% (or±1.67%).

In order to program the output frequency for a particular die, a dividevalue N is selected to reach the desired frequency (within the tuningrange). For a given frequency output desired, the control logic 109 cancount cycles in a given period of time and determine the VCO frequency(minimum and maximum). For example, assume the output of the VCO isnominally 4 GHz. A known clock can be supplied to the control logic, andthe control logic 109 counts VCO output cycles (or a divided downversion of the VCO output). Since the frequency of the clock supplied tothe control logic is known, the number of VCO cycles counted by thecontrol logic during a given time period, e.g. in the millisecond range,can be used to determine the VCO output frequency. It is then possibleto select a divider value (N) for the programmable divider 103 toachieve the target oscillation frequency so that the output frequency isclose to the desired one. That value of N can be stored in non-volatilememory (NVM). The non-volatile memory can be formed using any suitablenon-volatile storage technology, including one time programmable memory.However, even once N has been determined, the exact frequency may nothave been achieved, although the frequency determined by N is withintuning range Δf of the VCO. Thus, because the frequency is very closebut not exact, a fine “calibration” needs to be performed that canachieve the target frequency. In addition, it is also necessary toensure that the target frequency can be achieved at all temperatures. Toaccomplish temperature compensation, an open loop compensation approachis utilized as discussed herein.

Referring to FIG. 3, note that after defining the N value, theoscillation frequency is at point A, very close to the desired targetfrequency (at B). So in a factory environment, for example, thefrequency is steered from point A to point B by driving a digital wordthrough the DAC to change the VCO frequency. That is, the digital wordneeds to move the frequency to the target oscillation frequency at thefactory calibration temperature. The same approach for fine calibrationcan be used as described above for the calibration of N. That is, thecontrol logic 109 can count output cycles of the VCO over a known timeperiod until the desired frequency is achieved. Once that digitalcontrol word is determined for the VCO, the word is stored in NVM 111and the corresponding temperature as measured by the temperature sensoron chip is also stored. There are many other calibration approaches thatwould be readily apparent to those of skill in the art. For example, theoutput frequency could be measured externally and the divider value Nfor coarse tuning and the control value supplied to the DAC 105 for finetuning can be supplied over the serial interface. Note that thecalibration can be performed at two or more temperatures for even betteraccuracy.

In order to achieve a higher output clock accuracy than available fromthe free running VCO in an open loop configuration alone, an open looptemperature compensation scheme is utilized. Consider the profile offrequency vs. temperature vs. process in FIG. 3. As a function oftemperature, the integrated circuit will have a profile of temperatureand frequency. That profile is illustrated by either of the two curves301 and 303, which represent different process corners. That profile canbe characterized and stored in the NVM 111. In normal operation thetemperature sensor 113 senses the temperature and supplies an analogvalue representative of that temperature to analog to digital converter(ADC) 107. The ADC 107 supplies the digital representation of thetemperature to the control logic 109, which utilizes the digital valueto read the proper compensation factor out of NVM 111 and apply thatvalue to the DAC 105 to steer the VCO frequency to the right target.With this temperature compensation approach, the frequency should beconstant over temperature, or more precisely, constant within anexemplary target range of ±100 ppm. Note that the compensation schememay store a table of temperature vs. frequency compensation in thenon-volatile memory or/and store a fitting equation that allowscalculation of the temperature correction to be applied to the VCO.

Thus, during normal operation, open loop frequency compensation is usedto account for temperature variations. That is, during normal operation,the control logic supplies the appropriate value of the digital worddetermined during calibration to finely compensate to the desired outputfrequency along with any correction factors determined based on thesensed temperature to the DAC 105. That value is supplied to the freerunning VCO 101. In addition, the value of N stored in NVM is utilizedby the programmable divide by N circuit 103. Note that open loopfrequency compensation can be done at a relatively low frequency/ratesince it only needs to follow the temperature which is a slowly changingvariable. In one embodiment, the control logic 109 is implemented by amicrocontroller such as an 8052 microcontroller.

FIG. 4 illustrates a floor plan of an embodiment in which an 8052microcontroller is utilized for the control logic portion 109 ofoscillator circuit 100. The embodiment illustrated in FIG. 4 includespads 401, 402, 403, and 404 for SB (standby or enable), VDD, GND, andOUT, respectively. In addition, pads 405 are provided in the 8052microcontroller for programming/control of the 8052 through, e.g.,serial port 406. The digitally controlled oscillator includes inductor413 and the amplifier 415. The regulator 409 provides power regulationfor the VCO. The digital to analog converter for the control value forthe VCO is provided by delta sigma converter 417. The analog to digitalconverter for the temperature sensor 419 is provided by cyclic A/Dconverter 421. The use of the process sensor 423 is described furtherherein.

There are two important assumptions associated with the temperaturecompensation. The first is that the temperature/frequency compensationis a deterministic, i.e., not random, function. Indeed, this should bethe case since the frequency should only depend on physical parametersthat do not have “random” components. First order in an LC oscillator,for example, depends on the inductance L (geometrically determined) andon the capacitance C (mostly geometrically determined). So the mostimportant parameters end up being the thermal expansion coefficient ofmaterials. As an example, in certain implementations LC VCOs typicallyachieve on the order of 20 ppm/° C. in a very systematic manner, whichcan be compensated for with the temperature compensation schemedescribed above. Note that 20 ppm/° C. implies 2000 ppm/100° C. or ±1000ppm over 100° C. Assuming one can achieve compensation by a factor of 10using the temperature compensation scheme described herein, that leavesan uncompensated frequency error of about 1100 ppm over 100° C.

The second assumption is that the compensation is process insensitive.However, that might not be completely the case. For example, in an LC,the C will depend somewhat on the process corner. However, most of thedependency is still process insensitive. Referring again to FIG. 3, thefact that temperature sensitivity may be process dependent isillustrated by the two curves 301 and 303 for two process corners. Notethat while the illustrated curves intersect at the target frequency,other process curves that may be utilized may intersect at a differentlocation other than the target frequency or not intersect at all. If oneunderstands the physical dependence on the process corner (and often onedoes understand), then the temperature compensation approach can beextended to include a “process sensor” of the relevant parameter that isprocess dependent and use a somewhat more complex lookup table stored inNVM or some correction factors to improve the compensation code that maybe executed on the microcontroller. That is, process sensing can be usedto further refine frequency compensation based either on physicalunderstanding of frequency vs. process parameters or entirely on thebasis of an extensive characterization of the process.

In an exemplary embodiment, a process sensor (or sensors) 117 senses theone or more process parameters, e.g., one or more threshold voltages atnominal temperature. The sensed process parameters are utilized todetermine additional refinements to the temperature compensation. Otherembodiments may include additional sensors related to the capacitance(e.g., oxide associated with the capacitors), resistance ofinterconnects or circuit elements, mobility, or other process dependentparameters of interest. The particular sensors utilized depend on theprocesses utilized and the determination of the importance of correctingfor any particular process variation. By characterizing the appropriateprocess, the particular oscillator circuit can be temperaturecompensated more accurately, e.g., by compensating for temperaturechanges in accordance with the appropriate curve 301 or 303.

Process compensation is just one incremental improvement to the overallconcept to get even better frequency accuracy. Various embodiments mightdecide to use it or not, depending on the specific application orhardware implementation of the reference-less oscillator describedherein.

Thus, various embodiments have been described for implementing areference-less oscillator circuit. The description of the invention setforth herein is illustrative, and is not intended to limit the scope ofthe invention as set forth in the following claims. Other variations andmodifications of the embodiments disclosed herein, may be made based onthe description set forth herein, without departing from the scope ofthe invention as set forth in the following claims.

1. An integrated circuit for generating a clock signal comprising: acontrollable oscillator arranged in an open loop configuration; aprogrammable divider circuit coupled to divide an output of thecontrollable oscillator according to a programmable divide value togenerate the clock signal; control logic coupled to determine an outputfrequency of the controllable oscillator according a control word storedin a non-volatile storage and a detected temperature; and wherein thedivide value for the programmable divider circuit is stored in thenon-volatile storage.
 2. The integrated circuit as recited in claim 1wherein the integrated circuit is programmable to supply a frequency ina continuous frequency range, the controllable oscillator operable in afrequency range (fo) to (fo+Δf), where Δf represents the tuning range ofthe controllable oscillator and a minimum value for the divide value ofthe divider circuit is greater than or equal to fo/Δf.
 3. The integratedcircuit as recited in claim 1 wherein the integrated circuit isprogrammable to supply a frequency in a continuous frequency range, thecontinuous frequency range being at least 100 MHz.
 4. The integratedcircuit as recited in claim 1 wherein the controllable oscillatorcomprises one of an integrated inductor LC based voltage controlledoscillator circuit, an RC based voltage controlled oscillator and a ringoscillator.
 5. The integrated circuit as recited in claim 1 wherein theintegrated circuit includes a microcontroller.
 6. The integrated circuitas recited in claim 1 further comprising; temperature sensor to detect atemperature and supply an indication thereof; an analog to digitalconverter coupled to the temperature sensor and to supply a digitalindication of the detected temperature to the control logic.
 7. Theintegrated circuit as recited in claim 6 wherein the control logic iscoupled to adjust the output frequency of the controllable oscillatoraccording to the detected temperature.
 8. A reference-less oscillatorcomprising: a controllable oscillator circuit arranged in an open loopconfiguration; means for dividing an output of the controllableoscillator circuit according to a programmed divide value; anon-volatile storage storing a control word and the programmed dividevalue, a frequency of the output of the controllable oscillator circuitdetermined, at least in part, according to a value of the control word;and means for adjusting the output of the controllable oscillatorcircuit according to temperature.
 9. A method for operating anintegrated circuit to provide a reference-less oscillator comprising:determining a control signal for a controllable oscillator circuitarranged in an open loop configuration, at least in part, according to astored value in a non-volatile memory; controlling an output of thecontrollable oscillator circuit according to the control signal; storinga divide value for a divider circuit in the non-volatile memory; anddividing the output of the controllable oscillator circuit in thedivider circuit according to the divide value.
 10. The method as recitedin claim 9 further comprising adjusting the output of the controllableoscillator according to a temperature sensed in a temperature sensingcircuit.
 11. The method as recited in claim 9 wherein the controllableoscillator circuit can operate from a frequency of (fo) to (fo+Δf), Δfrepresents the tuning range of the controllable oscillator circuit, andwherein the minimum divider value (K−1) satisfies Δf/f₀≧1/(K−1).
 12. Amethod of making reference-less oscillator circuit that provides aselected output frequency comprising: programming a divide value for adivider circuit coupled to divide a first output signal from acontrollable oscillator circuit, a second output signal from the dividercircuit having a frequency that differs from the selected outputfrequency, wherein the controllable oscillator circuit can operate froma frequency of (fo) to (fo+Δf), Δf represents the tuning range of thecontrollable oscillator circuit, and wherein a minimum divide value(K−1) for the divider satisfies Δf/f₀≧1/(K−1); varying a control valuethat controls the first output signal from the controllable oscillatorcircuit until the second output signal from the divider circuit is theselected output frequency; storing a control signal corresponding togeneration of the selected output frequency in non-volatile memory;storing information in the non-volatile memory relating to adjusting thecontrol value according to a detected temperature; and storing thedivide value in the non-volatile memory.
 13. The method as recited inclaim 12 further comprising storing a temperature detected by atemperature sensor of the reference-less oscillator circuit at the timethe reference-less oscillator is supplying the selected outputfrequency.
 14. The method as recited in claim 12 wherein thereference-less oscillator circuit is programmable to supply a frequencyin a continuous frequency range, the continuous frequency range being atleast 100 MHz.